Problem: Region $R$ is enclosed by the $x$ -axis, the curve $y=\dfrac14 x^2-1$, and the line $x=4$. $y$ $x$ ${y=\dfrac14x^2-1}$ $ 3$ $ 2$ $ 4$ $ R$ What is the volume of the solid generated when $R$ is rotated about the $y$ -axis? Give an exact answer in terms of $\pi$.
Explanation: Let's imagine the solid is made out of many thin slices. Each slice is a cylinder with a hole in the middle, much like a washer. $y$ $x$ ${y=\dfrac14x^2-1}$ $ 3$ $ 2$ $ 4$ Let the width of each slice be $dy$, let the radius of the washer, as a function of $y$, be $r_1(y)$, and let the radius of the hole, as a function of $y$, be $r_2(y)$. Then, the volume of each slice is $\pi[(r_1(y))^2-(r_2(y))^2]\,dy$, and we can sum the volumes of infinitely many such slices with an infinitely small width using a definite integral: $\int_a^b \pi [(r_1(y))^2-(r_2(y))^2]\,dy$ We call this the washer method. What we now need is to figure out the expressions for $r_1(y)$ and $r_2(y)$ and the interval of integration. $r_1(y)$ is equal to the distance from line $x=4$ to the $y$ -axis. So, $r_1(y)=4}$. $r_2(y)$ is equal to the distance from the curve $y=\dfrac14x^2-1$ to the $y$ -axis. To find it, we need to solve the equation for $x$ : $x=\sqrt{4y+4}$ So, ${r_2(y)=\sqrt{4y+4}}$. Now we can find an expression for the area of the washer's base: $\begin{aligned} &\phantom{=} \pi [(r_1(y)})^2-({r_2(y)})^2] \\\\ &= \pi [({4})^2-({\sqrt{4y+4}})^2] \\\\ &=\pi\left[ 16-(4y+4) \right] \\\\ &=\pi(12-4y) \end{aligned}$ The bottom endpoint of $R$ is at $y=0$ and the top endpoint is at $y=3$. So the interval of integration is $[0,3]$. Now we can express the definite integral in its entirety! $\int_0^3 \left[\pi\left( 12-4y \right)\right]dy$ Let's evaluate the integral. $\int_0^3 \left[\pi\left( 12-4y \right)\right]dy=18\pi$ In conclusion, the volume of the solid is $18\pi$.